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ADDRESS 

"HEAT TREATING 
OF STEEL" 



BY 

FREDERICK J. NEWMAN 



NOTE 

Stenographically reported. Illustrations reduced from 
those shown during the address. 






ADDRESS, Delivered by Mr. Frederick J. 
Newman, President of Chicago Electric Motor Car 
Co., on the subject of "HEAT TREATING OF 
STEEL", before the Superintendents' and Fore- 
men's Club of the Chicago Branch of the National 
Metal Trades Association, at the Kuntz-Remmler's 
Restaurant, Chicago, on Saturday evening, June 
17th, 191 1. 

Mr. Chairman and Gentlemen : 

In taking up the subject of the heat treatment of steel, 
or rather, alloys of iron, we will consider briefly the metal- 
lography of iron and steel. By metallography we mean the 
study of the physical structure, that is, the grains, the crystals 
— anything that we can see under the microscope. It will not 
be necessary for us to consider metallurgy or chemistry, but 
metallography is a necessity. 

When we speak of seeing a metal under the microscope, 
we refer especially to our prepared sample that we are going 
to examine. We cannot take a fracture and look at it under a 
high power microscope, because in a fracture we would have 
high places and low places, and if we looked at it with a glass 
above 30 magnifications, we would probably have the moun- 
tains in focus, but the valleys would not be, so that in order 
for us to study the structure of steel under the microscope 
it is necessary for us to take our sample and polish it very 
highly, so that all the parts are on the same plane. After 
polishing we etch, that is, we apply a drop of acid and wipe 
it off quickly. The acid in that way brings out the demarca- 
tion line between the grains or crystals, or has different action 
upon different crystals. 

I want to state, however, at the outset, that I am not 
going to use language, possibly, that is absolutely scientifically 
correct. I am going to try to tell, it to you in a way that a 
user of steel might be able to understand. I am not going 



1 a - '*y%. 



2 Heat Treating of Steel 

to try to give it to you in language such as would require a 
college professor to fathom ; — not that you would not under- 
stand it, but I am afraid that I probably could not explain it 
properly, so I will take the responsibility of setting up a sub- 
stitute which we will call a "mixing theory." Some technical 
terms will be omitted, principally those which refer to transi- 
tion structures, so that instead of taking home with you a 
mixture of technical words you will have a clearer conception 
of the structure of steel as influenced by heat than you now 
have. 

When we speak of microscopic sections, we refer to a 
"pin-head size" of a piece of metal. Figure 1 in a photograph 
under fifty magnifications probably would be as large as a 
silver dollar, but as you see it here, represented by a linear 
reproduction, we probably have increased that about eight 
or ten times in diameter. While we show crystals clearly with 
a microscope of fifty magnifications, we do not show some of 
the things that a five hundred diameter microscope would 
show, but that figure is characteristic of the crystalline forma- 
tion of all pure metals. 

Metals will crystalize in a cubic way or something be- 
yond that, so that as we look at them in a single plane, we 
will see squares or hexagons. 

Each one of these grains is made up of a larger number 
of smaller grains, but in order to show up a condition of that 
sort, it requires very much more careful and deeper etching, 
and at the same time a very much higher power microscope 
than that from which our figures were taken. 

Figure 1 represents, as I said, the condition of pure 
metal. Now, pure iron is really only a laboratory product. 
Pure iron is not a commercial product. The nearest that we 
get to a pure iron is what we know as Norway or Swede's 
iron. Norway and Swede's iron has been understood or been 
considered for many years (and the theory is still held by 
those who do not know) as being a fibrous material, but since 
the microscope has been used in the study of materials, it has 
been found that wrought iron is not a fibrous material. If we 
would look at certain parts of the wrought iron, we would 

*h&jL****A mclcJL 3>u*-du* (Ww* 




Fig.| 1 — Pure Metal 

find exactly the crystalline structure shown in Figure 1, and 
then if we looked at other parts of it we would find a condi- 
tion as shown in Figure 3, and at other parts a condition as 
shown in Figure 4. 

Figure 2 represents a wrought iron bar. Figure 3 is a 
view taken from the end "A," while Figure 4 is taken from 




O 

3 




Fig. 2— Wrought Iron 



Heat Treating of Steel 




Fig. 3 — View A. Wrought Iron, Showing Slag Between Ferrite Grains 



the longitudinal side "B." Those black specks there are the 
end views of slag that has been rolled together with the iron. 
In the other figure the lines represent slag; in other words, 
the lines and specs are probably the projections of the same 
pieces, taken in another view. The slag that is mixed in and 
rolled out has given us the idea that wrought iron is a fibrous 
material. It is not. It is just as crystalline as any steel can 
be, but you will probably ask why do we not get it in a brittle 
condition as we do steel? I will explain that later on. 

Under the microscope sections of wrought iron are just 
as crystalline as, for instance, screw stock, and the reason 
that we think it is fibrous is due to the fact (as I have just 
stated), that slag has been rolled in with it. 

These crystals are known as "ferrite." Ferrite is the 
carbonless component, or carbonless crystal of iron or steel, 



Heat Treating of Steel 




Fig. 4— View B. 

and it is that component of iron or steel, or that particular 
crystal which has the particular property of weldability and 
ductility; it is also the softest crystal in iron or steel. 

Of course, you are all familiar with how wrought iron 
is made. It is puddled product. If we take iron and melt it 
and add some carbon and then pour it or cast it, we make 
steel. There is some carbon in wrought iron, and we can get 
the same chemical composition, that is, the same amount of 
iron, carbon and other chemical constituents, such as silicon, 
phosphorus, manganese, and so forth, in wrought iron as we 
can in low carbon steel, so that if we take two pieces — that is, 
one piece of steel and one piece of wrought iron that has 
exactly the same chemical make-up, the only difference be- 
tween the two would be the presence of slag in one and the 
absence in the other, brought about by the melting and sub- 
sequent pouring. 



6 Heat Treating of Steel 

I have here a few bolts that have been made of screw 
stock, which is probably the lowest carbon commercial, steel. 
The first one that we pass is characteristic of the brittle crys- 
talline structure of all carbon steels. The others have been 
given a heat treatment. One has been nicked and then bent, 
and the other has not been nicked. I am only showing these 
now to indicate that the apparent crystalline structure can be 
removed and that steel can be made to appear just as fibrous 
as wrought iron. 

As we add carbon we get another crystal, or another 
grain, properly speaking. I speak of these main divisions here 
as grains, and the finer ones as crystals. That grain is called 
"Pearlite." Pearlite is that grain which contains the carbon. 
It is a mixture of an iron carbide (cementite) and some ferrite ; 
in other words, as carbon is added we produce a carbide made 
up of a chemical union of a little iron and a little carbon, 
making the chemical formula Fe 3 C, and that mixed with a 
certain amount of the ferrite or carbonless portion gives us 
the pearlite grain. 

We have represented that as dark grain against light 
colored ferrite grain. Before I continue, however, I might 
state that in the sections of wrought iron there may be some 
ferrite, indicating that there was carbon ; under a small glass 
it may be confused with slag ; in other words, if there is some 
carbon in wrought iron, and the chances are there is, that 
there will be some pearlite. The reason that pearlite appears 
dark is this : 

Under high powers we see it laminated as in Figure 6. 
In Figure 5 I have shown the ferrite as being light colored, 
while in Figure 6 the ferrite as being dark colored, and the 
cementite being light colored. The carbide or cementite is 
hard. Ferrite is soft. As we polish the tendency is for the 
fine polishing rouge or polishing powder to polish the soft 
below the surface of the hard. When we etch, the tendency 
is for the soft to be below the surface of the hard. We can- 
not see it with the eye, we cannot detect it with the microscope 
ordinarily. It takes a trained eye to see it. The light in the 
microscope shines obliquely on the specimen. It is natural 



Heat Treating of Steel 




Fig. 5 — .10 Carbon Steel, Low Magnification 





Fig. 6— Pearlite Highly Magnified. 



8 Heat Treating of Steel 

that the high line blocks the light for the low one and puts it 
in the shadow so that the one that is up high will appear 
light, that is, it will be the natural color of steel or iron pol- 
ished, so that when we see, for instance, the light particles 
in the microscope, the chances are that they are high ones. 
The low ones are the dark ones, since they are in the shadow 
of the light ones. 

Now, that is the reason why pearlite then, as a whole, 
will be dark as compared to ferrite, because it has those 
shadow lines which influence the color of the whole grain. 
As we add carbon, the ferrite grains decrease and the pearlite 
grains increase. Figure 5 is characteristic of ten carbon steel, 
and Figure 7 is characteristic of fifty carbon steel. The in- 
creasing carbon gives us more of those dark places until we 
get up to a condition of eighty or ninety carbon, when we have 
pearlitic steel, that is, the whole mass becomes pearlite, and 
then the independent ferrite absolutely disappears. 
«r~7 As we go beyond that, then we begin to get free cementite. 

There is no free ferrite, because all of the carbide is combined 
with the iron to make the pearlite, and there is an excess of 
it, so we get out carbide in a separate and free crystal. For 
instance, Figure 8 would represent 130 point carbon steel. 
Here the cementite is white and the pearlite is dark. Again, 
the cementite as a whole being harder, stands up. If we 
increase that carbon up to 2 per cent, then we get cast iron. 
We have a tendency then towards the condition of having 
nothing but cementite. 

As we add carbon we increase the strength up to the 
condition of all pearlite, up to about 90 points, as we go be- 
yond that, we then get hardness and brittleness due to the 
carbide ; below, softness and ductility. When we get to a 
condition where -there is no more pearlite, it all becomes car- 
bide, then we have the extreme brittleness of cast iron. So 
we can consider the iron family as follows: 

Beginning with wrought iron and low carbon steel, the 
only difference being in the occluded slag. Next to low car- 
bon steel we have a medium carbon steel, then a high carbon 
steel, and then we get into cast iron. 



Heat Treating of Steel 




Fig. 7— .50 Carbon Steel 



We have three grain conditions that I have spoken of: 
one is ferrite, that is soft, ductile, and can be welded. We 
have the carbide condition, which gives us hardness and brittle- 
ness. Then we have the pearlite condition, which gives us a 
combination of both, and when we go so high as to get gray 
iron, we begin to get a free carbon condition, a condition 
higher than carbide, and then we get graphite. 

In general, I want to state the fact that as we heat to a 
high temperature, or as we cool from a high temperature, we 
promote the growth of crystals. The higher the temperature, 
the larger our crystals. That is a thing that I will want to 
take up later on in the evening. High temperature promotes 
the growth of crystals. Slow cooling from high temperatures 
that is, in pouring castings, also promotes the growth of 
crystals. 



10 



Heat Treating of Steel 




Fig. 8—1.30 Carbon Steel 
If we heat a piece of steel, we find* this: that the tem- 
perature would increase in a continuing amount, until we got 
to a condition around some of the red points, depending upon 
the content of the steel — where it refuses to get any hotter 
momentarily, although we continue to apply heat, and under 
some specific compositions the temperature will actually drop, 
although we still apply heat. That is only momentarily, and 
then the temperature goes up again. If we stop heating after 
we have reached this point, and allow the piece to cool, we 
find that the steel will gradually cool until its gets to a certain 
point, when the reverse takes place. That cooling point that 
I am speaking of is at a lower temperature than the point 
that I spoke of in heating. The piece cools until it gets to 
a certain point, no heat is being applied, but it stops cooling 
momentarily. In fact in some conditions it gets hotter, and 
then cools again until we get down to the cold temperature. 



Heat Treating of Steel 11 

I have here a steel wire by which I am going to illustrate 
that. Before I try the experiment I am going to tell you 
what is going to happen, so that you will watch for it. We 
will heat this wire by means of a storage battery which we 
have connected to it. The expansion or lengthening which 
takes place due to the heat, of course, will be noted by the 
sag. As long as that wire sags, it is taking on heat. The in- 
stant it stops sagging, although we continue to heat, it is not 
taking up heat ; in fact, if you watch it, you will notice that 
it will tend to rise a little bit, indicating that it is contracting 
and cooling, and then an instant after, you will notice that 
it continues to sag more, indicating it is taking up heat. When 
we get to that condition I will cut off the power and let it cool 
down, and you will notice that it is cooling, both by the con- 
traction and change of color until we get to a certain point, 
and then it will stop, and if you watch for it you will see it 
sag again, and then it will go up again as it cools. That is 
what the experiment is going to show, and we will discuss 
what happens when we get through illustrating. 

(At this point Mr. Newman had the room darkened and 
made the experiment referred to in his preceding remarks.) 

I suppose you all noticed that the change occurred and 
that the change on the cooling side, that is, after I cut off 
the power, was of a lower redness than on the heating side. 

That experiment shows only an outward demonstration 
of something that has occurred within the steel itself. Several 
things have occurred within the steel itself. That particular 
phenomenon is known as recalescence. There are only two 
things that we are going to consider in our theory as to what 
has happened. One is the carbon change and the other is 
the crystal form change. 

Truly speaking, there are three points in cooling and 
heating, but we are only going to consider one in cooling and 
one in heating. These we will call change points. 

Below the change point we have considered our carbon 
in the form of a carbide, a chemical combination with some 
iron, the amount depending upon the carbon, and we will 
know that carbon as "cement carbon." Above the change 



12 Heat Treating of Steel 

point the carbon is in an entirely different condition. It is 
known as "hardening carbon." Sometimes the carbon there 
is spoken of as "hardenite." 

To change from cement to hardening carbon requires 
heat. If heat is supplied as fast as it is needed then the 
temperature momentarily stops rising, but if more heat is 
required than is supplied, then the temperature actually drops. 
This takes place in heating. 

Now in cooling, the reverse takes place. Heat is liber- 
ated in changing from one carbon condition to another and 
the larger the amount of heat liberated, the clearer is the 
change point defined. When we mix chemicals together we 
get heat. In many conditions where we obtain a given chem- 
ical change, we either liberate or absorb heat. It is not neces- 
sary for us to explain why, but the chemists recognize it, and 
you all recognize that this condition exists. If a chemical 
change has taken place, it is reasonable to suppose also that 
the physical condition or change of the crystals has also taken 
place. 

I must ask you to imagine that when the change point 
has been passed in heating, that the carbon is dissolved in a 
matrix of plastic steel and free to float throughout the mass 
and intimately and uniformly distribute itself in this way. 
The crystalline formation under this condition is known as 
"Martensite." 

I want to call to your attention that there was a time 
element in which that change took place. That change was 
not instantaneous. If it were instantaneous you would not 
have been able to notice it in the experiment. It took some 
time for it to change from cement carbon to hardening carbon, 
and it took some time to change back from hardening carbon 
to cement carbon. Just on account of that is why we can 
retain that hardenite carbon condition if we quench it — in 
other words, if we cool it quicker than the time required for 
the change to take place. Then we hold in the cool material 
the crystal that we had in the hot material. That is, we get 
cgld martensite. That, I believe, is plain to understand, so 
that the rationale now for the hardening condition will be 



Heat Treating of Steel 



13 



this : That you must quench faster, or your quenching me- 
dium must take heat away quicker than the time required for 
the carbon change, otherwise the hardening carbon will have 
changed back again to cement carbon, and you would not 
change the condition of unhardened steel. 

Now, remember we changed as we went up from cement 
carbon to hardening carbon, and as we went down we changed 
from hardening carbon back to cement carbon, the condition 
of soft steel, and what does that teach us? That we must 
quench above that low condition, otherwise it would change 
back, and there nature helps us, because it has given us a 
range of temperature between heating and cooling during 
which time we have time to go from the fire to the quenching 
tank without overheating, a condition which would have to 
take place were the points coincident. 

If you quench before you get to that change point in 



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Fig. 9 — Martensite 



14 Heat Treating of Steel 

heating up, you get no hardening, because the change has 
not taken place, and if you quench after it has gone through 
that lower point, you get no hardening either, because the 
carbon is again changed from the hardening to the cement 
condition. 

Martensite, which is a well mixed condition, is shown in 
Figure 9 as a conglomerate structure, also has a specific phys- 
ical characteristic of its own, from the standpoint of strength. 

Martensite is the strongest and is the hardest crystal that 
we have in steel. Martensite also has the peculiar property 
of being very unstable when it is heated. If we heat mar- 
tensite up it tends to go back to a condition of ferrite and 
pearlite, and that gives us the rationale for our drawing, or 
reheating. You see as we heat up through the change point 
and quench we hold the martensite, but as soon as we reheat, 
we begin to get martensite of a lower degree, and begin to 
introduce some pearlite and some ferrite. After we get high 
enough, we find that we have destroyed all our martensite, and 
we have gone back to the condition of pearlite and ferrite. 

Let us now consider what has happened and what we 
have accomplished in heating, to just beyond the change 
point and quenching and then reheating. If you will refer 
to the various illustrations in which is shown ferrite and 
pearlite and even cementite, you will notice that there is no 
definite regularity with which the distribution of grains occur. 
You notice a number of- ferrite grains grouped together and 
a number of pearlite grains also grouped together. Also the 
condition may be such that the grains are large sized. Now. 
each grain has a definite physical characteristic of its own. 
One is strong and hard and brittle and another is tough, duc- 
tile and comparatively weak. Also the small sized crystals 
represent the best physical condition that the particular grain 
can be in. 

Martensite, as I stated before, represents the best mixed 
condition of all of the crystals, arid if quenched from a tem- 
perature close to the change point, it is fine grained. If we 
reheat to any point up to the upper change point, we do not 
destroy the well mixed condition, but retain that, and bring 



Heat Treating of Steel 15 

about a physical condition which represents the resultant of 
the specific physical, conditions of the specific grains. The 
higher we reheat, the more pearlite and ferrite we return to 
the structure and the more of their physical condition we 
impart, namely, toughness, ductility and softness. Without 
hardening and tempering and annealing we have areas having 
the physical properties of one grain and other areas having 
the physical properties of a different grain, but the heat treat- 
ment gives us the combination of both throughout the mass 
of the steel. 

To illustrate, we can take as an example the making of 
concrete. If we make a mixture of sand, cement, and of 
crushed stone and we do not mix it well, we can make a 
concrete wall ; but if we mix it well we can make a concrete 
wall which is a good deal stronger than one that was not 
mixed well. In each instance we get the advantage of cement, 
of sand and of stone, but we get their combined advantages 
in the best distributed form, and get the stronger and strong- 
est structure. 

Now, we have spoken of change points. It will inter- 
est many of you how to determine the change point, or in 
other words, the hardening heat. If you have a pyrometer, 
that is, a thermometer that will measure high heats (pyryo- 
meter simply means ''fire measure"), it is no particular trick 
to get it, because all you have to do is to make a record of 
temperature and time of heating, and the change points are 
readily observed. Then you can duplicate those temperatures 
in your forge. 

If you have to depend upon your eye, the following 
methods will give the change point and the heat is duplicated 
by color. 

For instance : You take a bar of steel and nick it in a 
half a dozen places to make breaking easier, as you will have 
to judge by fractures. Put one end in the fire and get that 
running towards a white heat condition, let the heat travel 
on out by conduction to a dull red and then quench it in 
water. Now break the piece at each nicked place and that 
fracture which is closest and finest grained was nearest to or 
at the change point. 



16 Heat Treating of Steel 

Now, of course, we are going to have steel, especially 
that in the lower carbons which will not show such a great 
difference in fracture with a slight difference in temperature, 
and in that case we can use another rough method. You know 
that if we take an ordinary compass, magnetic needle com- 
pass, showing north and south, and hold over it a piece of 
steel, that we can move that needle by the magnetic attraction. 
We also know that at the change point steel loses its ability 
to attract a magnet ; in other words, it becomes non-magnetic. 
When it goes through the change point in cooling, it begins 
to attract the magnet again. We can use that principle as 
follows : 

Lay a compass on a wooden bench, take your sample 
which is going to be your guide piece, hold it in the fire until 
it is red hot, take it out and wiggle it over the compass, and 
if the compass needle moves you are not high enough. Put 
it back again, heat it some more and repeat until, you get a 
condition where you cannot attract that needle any more, and 
then quench it. You find you have your material in the best 
condition as far as the grain is concerned. To duplicate the 
results, the color of the temperature will have to be remem- 
bered. Also you will need a brass tong for the test piece — 
iron tongs will cause the needle to move. 

As to the drawing methods, of course, you are all familiar 
with the method of taking your lathe tool, or chisel, heating 
it up to full redness, quenching the point and then with a 
brush scratch the end of it until it is brightened and let the 
heat run until you get the proper color, and then quench again. 

Of course, the quenching does not impart any hardness 
or any toughness. All. the quenching does under that condition 
is to stop the running of the heat, because if you do not quench 
and the other end is hot enough, it might cause too much 
drawing. 

I want to again emphasize that quenching below the 
change point does not effect hardness. It may, however, give 
cracking troubles. I much prefer an oil bath than color draw- 
ing. You can get in an oil tempering bath any condition you 



Heat Treating of Steel 17 

can get with the other method and with a great deal more ac- 
curacy. You do not run your heat over the blues. The bluest 
blues are something like 500 degrees Fahrenheit, which is 
below the flash point of tempering oil. 

There are many hand books which give color equivalents 
in temperature. Take an iron pot and put oil in it, put your 
piece in, take a thermometer that will register up to 650 de- 
grees and heat up until the temperature you are trying to 
obtain is registered on the thermometer, and then take it out 
and quench it or cool slowly. For temperatures above the flash 
point of oil, tempering can be done in melted lead and you 
can get a thermometer to read up to 1,000 degrees Fahrenheit. 

The next step in connection with drawing is annealing. 
Annealling really means the full conversion by heat of the 
martensite to the original grain condition, but in the inti- 
mately mixed condition as spoken of in the concrete wall 
illustration. 

As to just what treatment you want in the particular 
piece, that depends, of course, upon the material, and also the 
use to which it is put. 

If you desire machining qualities only and intend to heat, 
treat after machining, heating to full redness and allowing to 
cool slowly is all that is necessary. If no treatment is given 
after machining, heating to the change point, and in general 
for soft steels, to full redness and quenching in water and 
then reheating will put the steel in good condition. 

As to the reheating — the amount all depends upon the 
hardness you can stand. If wanted very soft, a full red heat 
is necessary with slow cooling. A little more hardness and 
a good deal of toughness is obtained by reheating to a low 
red heat and then quenching or allowing to cool slowly. I 
prefer water quenching after the reheat, as it is a time and 
floor space saver. I have also noted that in ordinary forgings 
this second method of reheating, although making a greater 
apparent hardness than the unheat treated forgings, gives a 
freer machining piece and one uniformly hard, so that there 
is no chatter, especially in lathe operations. 

I always prefer water quenching to oil quenching if the 



18 Heat Treating of Steel 

shape of the piece will stand it without cracking. It is cleaner 
and cheaper and you don't have to bother about keeping the 
bath cool. 

I have here a number of samples, all of which have been 
given a simple treatment which I will describe later on. These 
three screws were all made from ordinary cold rolled screw 
stock. The one not heat treated shows its brittleness. The 
other two are bent — one is nicked, the other is not — you will 
note the toughness. 

This is a piece of screw stock. One end heat treated, 
the other is not. You will note how apparently brittle the 
unheated end is. This forging is of 25 carbon open hearth 
steel. There was no great difficulty in breaking it, while the 
other forging after the heat, treatment, was nicked and bent, 
but did not crack. This piece of steel is from a bar of 50 
carbon open hearth. Note the two ends. The one which is 
broken and looks brittle was not heat treated. The bent end 
had the same heat treatment as the other specimens. A glance 
at it will, show how tough it is. This piece of material with 
the treatment will make a fine spindle for a lathe or any place 
where a tough stiff shaft is wanted. This treatment can be 
given to all forgings up to 50 carbon steel — spindle stock, 
screw stock and the like — before machining without destroying 
the free machining qualities. It is advisable, however, in 
making a large number of pieces from screw stock, and espe- 
cially where the parts are small, to give the treatment to the 
piece rather than bar, because I imagine you would not have 
furnace facilities for treating a long bar. 

I also find that chrome nickel and vanadium steel forgings 
can be given this treatment before machining, without making 
machining unusually difficult; in fact, in view of the straight- 
ening and cleaning necessary after heat treatment, it is better 
to treat before machining, even with these alloys. 

•(Mr. Newman here exhibited several samples of forging 
to the audience.) 

The heat treatment is as follows : 

Heat to full redness and quench in water. Then heat 
again to what is called a black heat — nascent red — the red 



Heat Treating of Steel 19 

that you can just see in the dark corner of a blacksmith shop, 
and quench again. If you are bothered with hardness in 
machining", you can heat a little hotter on the reheat, but my 
advice is to hold that heat as low as you can and get free 
machining. This is a universally good treatment for struc- 
tural steel, and one that is very simple. Of course, there is 
a limit to the size of piece which will give good results under 
heat treatment, or rather as good results as in a smaller sec- 
tion. The limit, of course, depends upon whether we can cool 
quicker than the time required for hardening carbon to change 
back to cement carbon. In any event, however, the piece will 
be improved, whether one inch thick or four inches in diameter. 

One more word in regard to hardening carbon tool steels. 
If you find that after water quenching from the change point 
you do not g"et the hardness you want, don't make the mistake 
of increasing your heat. If you can't get the hardness in 
water, the chances are that the piece was of large section, and 
the inside still hot had a drawing tendency on the outside 
parts, making it softer than you wanted, but increased its 
toughness. In this case you had better consult a good steel 
man and get the material that will harden. 

You will also understand from what has been said that 
all steels will harden in water, but the kind of steel and the 
shape of the piece may require a slower cooling medium to 
prevent cracking. 

We know now what heat treatment can do. The natural 
question is, why is heat treating necessary? In other words, 
what brings about the original hardness, what brings about 
the original brittleness in steel as we get in forgings and 
bars? I said to you before that high temperature induces 
large crystals, and large crystals represent brittleness. If 
you consider that the steel as we get it was poured from the 
highest temperature that we will ever put into steel ; add to 
that cooling probably slowly (which induces the growth of 
crystals), and you will appreciate that we have induced into 
that material the largest possible crystals that we will ever 
be able to induce. Furthermore, in the rolling operation, and 
in the hammering operation in the mill., and also in the ham- 



20 Heat Treating of Steel 

mering operation in forge, we have done all of these at a 
temperature way above the change point. We have not given 
any heat treatment to cure it, so to speak, consequently we 
retain the large crystals, the brittle structure that we orig- 
inally had. 

As far as hardness is concerned, you will understand 
that all steels have more or less an air hardening property — 
that is, the air cools the red hot piece quicker than the time 
required to go through the change point, consequently it 
is not surprising to get bar steel, that is hard. 

The casting proposition in the mill also suggests the 
question of the brittleness and heat treating of steel castings. 
Steel castings are true steel, except that they have not had 
the structures refined in them through hammering operations, 
but their brittleness is due to the high temperature from 
which they were poured. You can convert these castings just 
the same as you can convert any other steel, by rational heat 
treatment. Simple annealing will help the castings mate- 
rially, but for small castings the double heating and quench- 
ing treatment is recommended. 

Now then, the next step in heating is burned steel. 
While you can convert overheated steel, you can never con- 
vert burned steel, because burned steel has been put into a 
condition in which the chemical constituents are changed. 
Something has been burned out of it that has given it life. 
The condition has been changed, you have an entirely differ- 
net mass, and you can never convert it. The best thing to 
do is to scrap it. 

In regard to the heat treatment of cast iron, that takes 
in a little bit different explanation than our general mixing 
theory. If we take white cast iron we find that it is made up 
of cementite. Cementite, as I said to you, is the highest car- 
bon condition. If that carbide is exposed to high temper- 
ature, that is, a temperature above the change point, for any 
length of time, the tendency is for it to break up, and some 
of that carbon comes from the cementite and gives us a pure 
graphite condition. If we allow it to remain at this temper- 
ature long enough we get a pearlite condition and a ferrite 



Heat Treating of Steel 



21 



condition. This starts from the outside and works in, and 
we have in good malleable castings — that is, heat treated cast 
iron — a case made up largely of ferrite, the grain of wrought 
iron in which are globules of graphite, but the structure as 
a whole tends to the characteristics of the ferrite and pos- 
sibly ferrite and pearlite grains. 

Now, we can have exactly the same condition as far as 
chemical structure is concerned in a brittle gray iron as we 
have in the shell of a malleable casting. We will then have 
the grain make up; that is, we can have the same amount of 
ferrite and pearlite and under these conditions graphite. 




Fig. 10— Malleable Casting Shell, Ferrite- White, Pearlite-Black, Graph- 
ite-Black Spots 

In the case of a malleable casting we have ductility and 
malleability, and the gray iron casting is very brittle. The 
microscope has shown us that the difference between the two 
is only in the arrangement of the graphite. Figures 10 and 



22 



Heat Treating of Steel 




Fig. 11.— Gray Cast Iron, Ferrite- White, Pearlite-Black, Graphite-Lenis 

11 are characteristic. Eleven shows a gray iron structure. 
Throughout the entire mass we have those broad bands, or 
plates, or ribbons, you might say (they run across and length- 
wise), and they break up the entire mass. In a malleabilized 
casting, or take that portion of the piece which has been 
truly malleabilized, the graphite has segregated into little 
circular or globular shapes — in other words, to use common 
language, the cast iron would be represented as having a 
whole lot of cracks throughout the mass, microscopic cracks, 
in which the graphite has been placed, while a malleable 
casting has a large number of fine microscopic pin holes in 
which the carbon graphite was placed. 

It is only through the difference in the graphite condi- 
tion that we can really explain the difference between the 
brittleness of the cast iron and the apparent malleability of 
the malleable casting. The bands in the cast iron has broken 



Heat Treating of Steel 23 

up the continuity of the mass. You can readily understand 
the difference between a casting full of cracks and one full 
of pin holes. 

There is one more point, and that is case hardening. 
When we case harden we are adding a tool steel case to a 
softer center, in order that we can get the hardness of a tool 
steel, and also get the toughness and strength of the center 
of the softer steel. Now, if we heat beyond the change point 
and expose the steel to a carbon atmosphere — for instance, 
by packing in bone black or heating in cyanide or any mate- 
rial that will give off carbon at high heat, the carbon will 
flow in to our steel mass. The longer it is exposed the more 
carbon is taken up, and the microscope also shows us that 
the greatest amount of carbon is at the outside of the case, 
and the lowest amount on the inside; that is, as we increase 
the depth we also increase the amount of carbon on the out- 
side. 

Let us see what takes place. As we increase carbon we 
decrease the temperature or location of the change point, so 
that after adding a tool steel case we have a mass which has 
a lower change point on the outside than on the inside. We 
have, however, heated to the change point of inside to cause 
it to take up carbon. We also know that to get quick casing 
a still higher temperature is necessary, and it is customary 
to perform the casing operation at a temperature consider- 
ably above that of the change point. Now, after casing, if 
we quenched, you can readily appreciate that the case would 
not be in its finest grained condition, and if the temperature 
were high, even the core might be very brittle. It is pos- 
sible by several quenches to properly adjust the quenching 
temperatures, so that a hard and fine grained shell can be 
obtained with a tough and comparatively soft core, which 
really accomplishes the object of the case hardening operation. 

To get good hardening it is necessary to cool slowly after 
the casing operation. 

If you reheat to a temperature between that of the 
change point of the case and the core and quench you can 
get very good results. The best results combining toughness 



24 Heat Treating of Steel 

and hardness, however, require two quenches. After the slow 
cooling from the casing fire, reheat to the change point of 
the core and quench. Then reheat to the change point of the 
case and quench again. The first quench refines the core and 
the second refines the case and anneals the core. To put it 
into language referring to grain structure, the first quench 
gives martensite throughout the mass — fine grained on the 
inside and coarse on the outside. The second reheat changes 
the inside to pearlite and ferrite and the quench has no effect 
since the inside was below the change point, but the outside 
has had fine grained martensite fixed throughout by the 
quenching operation. 

I have here three specimens of case hardening. I will 
pass them around. All three were heated in a casing box 
at full red heat for several hours. The material is 25 car- 
bon open hearth. One sample was quenched directly after 
casing, the other two were slowly cooled. This one sample 
shows no definite structure between the core and case and 
the break shows it brittle (referring to the one quenched 
directly). 

The next sample was reheated to the change point of 
the core and quenched and broken. You will note that it is 
also brittle, but even though the case is not as clearly defined 
as in the following sample, still the core is finer grained than 
the first sample. The third piece had the same operation put 
to it as the second, and in addition was reheated to the 
change point of the case and quenched. Note that this piece 
is bent and broken only part through, an indication of its 
toughness, and at the same time I want you to notice the 
fine grain of the case and the apparent fibrous structure of 
core. The exact operation is as follows : 

Heated for three hours in cyanide at 900 degrees Centi- 
grade and cooled slowly. Reheated to 800 degrees Centi- 
grade and water quenched, and then reheated to 720 degrees 
Centigrade and water quenched. 

That is all I have to say, gentlemen, but I will be very 
glad to answer any questions that may occur to you. 

I thank you. 



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